FGF-21 is an endocrine hormone that is naturally found as a monomeric non-glycosylated protein. Together with FGF-19 and FGF-23, FGF-21 belongs to the endocrine-acting sub-family while the remaining of the 18 mammalian FGF ligands are grouped into five paracrine-acting sub-families. Endocrine-acting FGFs, in contrast to paracrine-acting FGFs, exhibit only low affinity for heparin-sulfate and are thus able to enter the blood circulation. Thereby, endocrine FGFs are able to regulate metabolic processes, such as bile acid homeostasis, hepatic glucose and protein metabolism (FGF-19), glucose and lipid metabolism (FGF-21) and vitamin D and phosphate homeostasis (FGF-23).